The European Union, United States and other allies have accused Russia of fuelling the conflict in eastern Ukraine and imposed wide-ranging sanctions on Moscow. For its part, Russia has denied involvement in the fighting and imposed a ban on Western food imports.

"Nato partners have provided assistance for the operation of maritime patrol aircraft from RAF Lossiemouth for a limited period with the Royal Navy. We do not discuss the detail of maritime operations," the UK's MoD said in a statement.

The submarine's periscope was spotted in waters usually frequented by British vessels as they enter or leave the Royal Navy's Faslane submarine base.